We experienced a rare case of thoracic outlet syndrome caused by hypertrophic nonunion of the first rib. A diagnosis was made mainly upon provocative tests and imaging studies. Pain and tingling could be reproduced and the radial pulse obliterated by the hyperabduction test. Abundant callus formation on the posterior aspect of the first rib with fracture line was visible on plain radiograph. Two-dimensional computed tomography showed right thoracic outlet narrowing mainly caused by the mass-effect of the callus. Dynamic arteriographic studies revealed an external compression of the right subclavian artery and duplex ultrasonography demonstrated a reduction in right subclavian artery blood flow when the shoulder is in 90 degrees of abduction. Surgery was performed after the conservative management for three months which failed to relieve the patient of his complaints. Resection of the first rib via transaxillary approach was undergone uneventfully in combination with the myotomy of the scalenus anticus muscle. At postoperative one year follow up, the patient was free of symptoms, and had a full range of motion of the right shoulder with no evidence of arterial insufficiency.